a) Field of the Invention
The present invention relates to optical devices formed by metallic particles suspended in a medium, and more particularly, to such devices which exhibit third order non-linear susceptibility.
b) Summary of the Prior Art
Nonlinear optical materials are central to optical domain computing as real time holographic and bistable memory devices, as optical correlator devices, as phase conjugator devices and as thresholding devices. A number of fast response time polymeric and semiconductor materials have emerged that have electronic nonlinear mechanisms in the picosecond and subpicosecond time scale. However the small magnitude of the optical nonlinearity requires a high laser density to utilize these materials. The consequent power dissipation limits the utilization of these materials.
Optical phase conjugation has been measured from dilute linear suspensions of nonlinear nanospheres in degenerate four wave mixing experiments. In "Phase Conjugation in Liquid Suspensions of Microspheres in the Diffusive Limit," 31 Phys. Rev. A. 2375 (1985), Rogovin and Sari attributed the apparent third order optical susceptibility, X.sup.(3), to a slow electrostrictive mechanism for dielectric spheres in a dilute suspension. See also Smith et al., "Four-wave Mixing in an Artificial Kerr Medium," 6 Optics Letters 284 (1985), and Neeves et al., "Polarization Selective Optical phase Conjugation in a Kerr-like Medium," 5 Opt. Soc. Am., B 701 (1988). These suspensions suffer from slow response time and grating instabilities that give rise to a poor signal to noise ratio.
On the other hand, in "Optical Nonlinearities of Small Metal Particles: Surface-mediated Resonance and Quantum Size Effect," 3 J. Opt. Soc. Am. B 1647 (1986), Hache et al. attributed the apparent .chi..sup.(3) of metal spheres in a dilute suspension in a linear dielectric medium to a fast electronic mechanism. See also Ricard et al. "Surface-mediated Enhancement of Optical Phase Conjugation in Metal Colloids," 10 Optics Letters 511 (1985). For this case the effective .chi..sup.(3) is enhanced at the surface mediated plasmon resonance frequency.
Some metallic suspensions therefore have a fast response and an effective optical nonlinearity with large magnitude. However, due to the large dielectric loss, the figure of merit for nonlinear composite materials is relatively poor. The figure of merit is defined as the ratio of .chi..sup.(3) to the optical absorption for the material.